DESCRIPTION: (Verbatim from the applicant's abstract) Dorsoventral polarity of the Drosophila embryo is established by local activation of the Toll signaling pathway. The membrane receptor Toll appears to be activated by an extracellular ligand that is localized to the ventral side of the embryo. The Toll ligand is generated by a proteolytic processing reaction which requires four members of the serine protease family encoded by the nudel, gastrulation defective (gd), snake, and easter genes. Earlier genetic studies suggested that these proteases act in a sequential activation cascade in which the final protease processes the Toll ligand precursor, and that activation of proteases in this cascade is ventrally localized, thereby spatially restricting production of the Toll ligand. Our more recent studies suggest that the pathway producing the Toll ligand is regulated at multiple steps so that the Toll ligand is generated at an appropriate level and at a defined time and space during embryonic development. Our major goal is to understand how the proteases required to produce the Toll ligand are organized into a pathway and how they are activated and regulated. Our specific aims are: 1) To test if the proteases act in a sequential activation cascade and if the activation of any of the proteases is spatially regulated, by biochemica1 analysis of the proteases in vivo; 2) To determine if the proteases are activated on the embryo surface and if activation is spatially restricted, using immunolocalization methods; 3) To decipher the role of the GD protein in the pathway, through a combination of molecular genetic and biochemical experiments; and 4) To identify factors that spatially control the activity of the Snake and Easter proteases, using biochemical and genetic approaches. Recent studies suggest that the ancestral role of the Toll signaling pathway is in host defense, to recognize and signal the presence of microbial pathogens, and is shared by the immune systems of insects and mammals. The signal that acts as the ligand for Toll in mammalian immunity, or how this signal is generated, is not known. Thus, the information obtained from our proposed studies should be of significant relevance for addressing the question of how the Toll signaling pathway is activated during the immune response.